This invention relates to a high-temperature pressure sensor element, to a component for power units, and to a method of producing a high-temperature pressure sensor element. The high-temperature pressure sensor element is particularly suitable for measuring pressures inside power units.
Pressure sensors are used in different technological fields for measuring pressures of gases and liquids. In many cases, the pressure sensors are exposed to particularly high strain, which depends on the state of the medium in which the measuring of pressure takes place. The pressures acting upon the pressure sensor often vary considerably. A pressure sensor therefore has to withstand high strain while supplying precise measuring results.
Particularly with respect to measurements inside power units, for example jet engines of airplanes or rocket engines, the pressure sensor has to withstand very high temperatures and to have very low errors or measuring inaccuracies, despite extreme environmental conditions, which are subject to high fluctuations. This also applies to other uses, such as inside motors and other internal-combustion engines, etc.
A known pressure sensor has a membrane which deforms in the event of a pressure difference on both sides of the membrane. The deformation of the membrane is measured, for example, by piezo-resistive or piezo-electric elements arranged on one side of the membrane.
Particularly in the case of high temperature-caused strains, it is problematic when the membrane of the pressure sensor deforms or is distorted in its frame or its suspension. The results are inaccurate measurements or falsified measuring results, which occur particularly during large temperature fluctuations. Furthermore, with classical micromechanical silicon membranes, there is the problem of plastic deformation at high temperatures.
Housings frequently consist of metal with high thermal coefficients of expansion, while the sensor elements and their components are made of silicon, silicon carbide, ceramic materials or the like, with coefficients of expansion that are clearly less than those of metals. As a result, mechanical deformations occur during operation at high temperatures because of the different expansions, which deformations propagate onto the membranes. These thermally induced tensions falsify the measuring signals.
German Patent Document DE 196 44 830 C1 shows a pressure sensor having a housing with an interior that is closed by a membrane, and piezo-electric elements which generate a corresponding signal when the membrane is deformed. By way of an additional flexible measuring element, which is coupled to the membrane and with a deformation that is measured, the measuring results are not falsified by deformations of the membrane, and precise and reliable measurements can be carried out, even when the temperatures fluctuate considerably. However, solutions of this type have the disadvantage of relatively high constructive expenditures.
For measuring deformations at high temperatures, indium tin oxides, for example, are suitable, as described in the articles “High Temperature Stability of Indium Tin Oxide Thin Films”, Otto J. Gregory et al., Thin Solid Films 406 (2002), 286-293, and “A Self-Compensated Ceramic Strain Gauge for Use at Elevated Temperatures”, Otto J. Gregory, Q. Luo, Sensors and Actuators A88 (2001), 234-240.
It is an object of the present invention to provide a high-temperature pressure sensor that is suitable for measuring pressures at temperatures of up to far above 400° C., as they prevail, for example, in airplane engines, and, in the process, supply precise measuring results while extending service life. Furthermore, a method of producing such a high-temperature pressure sensor is to be provided.
The high-temperature pressure sensor according to the invention is particularly suitable for power units and comprises a substrate in which an interior is developed, a deformable membrane, which separates the interior space from the exterior space during operation and that deforms when the exterior pressure changes, and a strain measuring element, which is arranged on the membrane, for measuring the deformation of the membrane. The substrate, the membrane, and the strain measuring element are all manufactured of the same material.
By way of the invention, measurements of pressures can take place at temperatures far above 400° C., for example, at approximately 1,000° C. Furthermore, particularly at lower temperatures, the service life is extended in comparison with previously known pressure sensors. A pressure sensor according to the invention is therefore also particularly suitable for use in airplane and rocket engines. The high-temperature pressure sensor element has a particularly high temperature stability because the materials of the components are the same, and therefore deformations do not occur as a result of different coefficients of expansion. As a result, the strain measuring element, just on the basis of a temperature change of the sensitive layer, expands no differently than the substrate or the carrier and thereby causes no deformations.
According to the present invention, with the exception of the insulator layer, all components of the sensor element are produced from the same material, namely a high-temperature-stable metal alloy. In addition, the housing may also be made of this metal. As a result, thermally induced deformations are minimized. In addition, the high-temperature-stable metal alloy is resistant in many aggressive atmospheres and, also at a high temperature, is still elastic over a wide strain range. Furthermore, the specific resistance is only slightly dependent on the temperature. The high-temperature-stable nickel base alloy Haynes 230, used for power units and housings, is preferably used as a sputter target in order to use this material as a sputtered thin film for this micro-system-related use.
The components of the pressure sensor element and the housing are preferably made of the same material as a power unit wall into which the housing can be screwed.
The strain measuring element advantageously is a sensitive layer, and preferably is a strain gauge in the form of a thin-film strip conductor. This results in simplified, rapid, and cost-effective manufacturing.
The deformable membrane is constructed in one piece with the substrate. This results in an even more improved high-temperature stability and in lower deformations. It is an additional advantage that the high-temperature pressure sensor element can be produced by micromechanics techniques.
Advantageously, the high-temperature pressure sensor element is useable for integration in a turbine element, such as a turbine blade. This is a result of permitting the high-temperature pressure sensor element to be manufactured with an extremely small construction and to be used without a housing. For example, the interior of the substrate may only be completely closed because of the integration in the turbine blade or by way of a partial surface of the turbine blade.
It is also conceivable to hermetically close off the interior by a seal that is also manufactured from the same high-temperature-stable material and is connected with the substrate, for example, by welding, and in particular, by electron beam welding. As a result, a reference pressure can be generated behind the membrane or in the interior of the substrate, and the interior is evacuated.
Advantageously, an insulation layer, which is used for the electric insulation, is constructed between the membrane and the strain measuring element. The insulation layer is formed, for example, by BN, MgO, Al2O3, or a combination thereof.
The insulation layer is preferably applied to the membrane by sputtering. However, it can also be applied by a sol-gel process or by vapor depositing, and can be further optimized by oxidation or annealing.
Optionally, a passivation layer can also be applied over the strain measuring element, which layer consists of the same material and can be applied by the same method as the insulation layer. As a result, the service life of the strain measuring element is further increased.
The high-temperature pressure sensor element is advantageously used in a high-temperature pressure sensor for power units, particularly airplane and rocket engines. Even under very rough environmental conditions, the prevailing pressures can thereby be determined in a precise and reliable manner. In this case, the high-temperature pressure sensor element is particularly preferably integrated in a turbine blade.
According to one aspect of the invention, a method of producing a high-temperature pressure sensor has the steps of providing a substrate, applying an insulation layer to the substrate, applying a strain measuring element over the insulation layer, optionally applying a passivation layer made of the same material and by means of the same method as in the case of the insulation layer, and producing a deformable membrane from the partial area of the substrate. Subsequently, the strain measuring element is arranged on the membrane in order to measure a deformation of the membrane. The substrate, the membrane, and the strain measuring element are produced of the same high-temperature-stable material.
The deformable membrane is advantageously worked out of the substrate from its back, so that a recess is formed in the substrate.
The substrate can be shaped such that an interior space is developed on the back of the membrane, which interior space is hermetically closed off in the measuring operation.
The invention is illustrated by way of example in the drawings.